Since their commercial appearance in the 1960's, Light Emitting Diodes (LED) have become ubiquitous in electronic devices. Traditionally, LED light output was ideal for indicator applications but insufficient for general illumination. However, in recent years a great advance in the development of high-intensity LEDs has occurred. These new LEDs operate at much higher current levels than their predecessors (350 milliamps to several amperes compared to the 10-50 milliamp range for traditional LEDs). These new power LEDs produce sufficient output current to make them practical as sources of illumination.
Presently, the high cost of the new power LEDs renders them best suited for applications where the unique characteristics of LEDs (ruggedness, long life, etc.) compensate for the extra expense. However, the cost of these high power LEDs continues to fall while efficiency (luminous flux generated per unit of electrical power consumed) continues to rise. Predictions are that in the near future, LEDs will be the source for general illumination, preferred over incandescent, florescent lamps or the like.
LEDs are a type of semiconductor device requiring direct current (DC) for operation. Since the electrical power grid delivers alternating current (AC), a line-powered device must convert the AC to DC in order to power the LEDs. Another increasingly common requirement for line-operated equipment is power factor correction (PFC, also referred to as “power factor control”). Devices which are capable of power factor correction are able to maximize the efficiency of the power grid by making the load “seen” by the power grid appear (approximately) purely resistive thus minimizing the reactive power. The efficiency of resistive loads arises from the unvarying proportionality between the instantaneous voltage and the instantaneous current.
Furthermore, LEDs are current driven rather than voltage driven devices. Therefore, the driver circuit usually regulates the load current of the LED device more precisely than the voltage supplied to the device terminals. The need for current regulation imposes special considerations in the design of LED power supplies since most power supplies are designed to regulate output voltage. Indeed, the design of the majority of integrated circuits (IC) commercially available for controlling power supplies is for voltage regulation.
For safety, it is desirable for the output of the power circuit (connected to the LEDs) to include galvanic isolation from the input circuit (connected to the utility power grid). The isolation averts possible current draw from the input source in the event of a short circuit on the output and should be a design requirement. Usually, optocouples are used to galvanically isolate a feedback signal representing the regulated output current from the input circuit of the power supply circuit. The power conversion is accomplished by using a transformer.
Another design goal for the conversion from the incoming AC line power to the regulated DC output current may be accomplished through a single conversion step which is controlled by one switching power semiconductor. A one-step conversion maximizes circuit efficiency, reduces cost, and raises overall reliability. Switching power conversion in the circuit design is necessary but not sufficient to satisfy the one-step conversion requirement while capitalizing on the inherent efficiency.
There is a need for a LED power supply circuit that provides a high power factor as well as a regulated output current while not requiring any feedback signals to be tapped at the current output. Thus, optocouplers or similar components, which are usually employed for transmitting the current feedback signal back to the input circuit while providing a galvanic isolation, can be omitted.